1. Field of Invention
The present invention relates to a TMSI (temporary mobile station identity) allocation scheme in a mobile communication, and particularly to a layered TMSI allocation scheme for a M2M (Machine-to-Machine) communication.
2. Description of Prior Art
A M2M communication is identified as a form of data communication which involves one or more entities that do not necessarily need human interaction. MTC (Machine Type Communication) services may involve a potentially very large number of communicating terminals, i.e. MTC devices, which is different from services that the current mobile network communication may provide.
The large number of MTC devices means the large addressing space requirement, which may go beyond a limitation that is defined for H2H (human-to-human) communications. When the MTC device is in a RRC (Radio Resource Control)-idle state, TMSI and IMSI (International Mobile Station Identity) are stored in a MME (Mobility Management Entity) or a SGSN (GPRS Serving Supporting Node) for paging the MTC device when a network-originated call comes. However, IMSI is a unique and fixed identity for the MTC device; it is very dangerous to broadcast IMSI at the air interface because of the possible track by enemies. Compared with IMSI, employing TMSI in the paging message is a relatively secure approach. TMSI is 32 bits long, whose allocation may be limited by the following factors.                In order to avoid double allocation of TMSIs after a restart of an allocating node, some part of the TMSI may be related to the time when it is allocated or contain a bit field which is changed when the allocating node has recovered from the restart.        In areas where both MSC-based circuit-switching services and SGSN-based packet-switching services are provided, some discrimination is needed between the allocation of TMSIs for MSC-based circuit-switching services and the allocation of TMSIs for SGSN-based packet-switching services. The discrimination should be done on 2 most significant bits, with values 00, 01, and 10 being used by the VLR (Visit Location Register) and 11 being used by the SGSN.        If an intra-domain connection of RAN (Radio Access Network) nodes to multiple CN (Core Network) nodes as described in 3GPP TS 23.236 is applied in the MSC/VLR or SGSN, a NRI (Network Resource Identifier) should be a part of the TMSI. The NRI has a configurable length of 0 to 10 bits. A configurable length of 0 bits indicates that the NRI is not used and this feature is not applied in the MSC/VLR or SGSN. 10 bits of the NRI should be mapped to bits 23 to 14 from a highest bit to a lowest bit. For the NRI with N<10 bits, the Nth bit in a NRI field should be mapped to bit 23 of the TMSI, and the (N−1)th bit in the NRI field should be mapped to bit 22 of the TMSI, and so on.        
After considering the aforementioned items, the available TMSI space will be reduced greatly. Thus, a problem of a potential TMSI identity exhaustion for the large number of the MTC devices is raised. A feasible TMSI allocation method capable of satisfying deployment requirements of the large number of the MTC devices in M2M communications is desired.